This invention relates to a variable-gain optical equalizer that equalizes the light intensity of optical signals at different wavelengths, and to optical gain equalizer equipment that uses this equalizer.
Wavelength division multiplexing (WDM) optical transmission is a key technology for increasing the capacity of optical communication equipment. In this transmission scheme, a plurality of optical signals at different wavelengths are multiplexed together and transmitted in a single optical transmission line. Also, technologies that have been developed and put into practical use for optical amplifiers include optical semiconductor amplifiers that use optical semiconductors, and optical fiber amplifiers in which the amplification medium consists of an optical fiber doped with a rare earth material such as erbium. Since these optical amplifiers are able to simultaneously amplify optical signals over a range of wavelengths where a positive gain can be obtained, they can be applied to WDM optical transmission equipment to implement high-capacity long-distance transmission equipment.
However, the optical gain characteristics of an optical amplifier are dependent on wavelength. This wavelength-dependence gives rise to non-uniformity in the light intensity of each optical signal (referred to as “wavelength imbalance” in the following). A concerted effort at reducing this wavelength imbalance will lead not only to a broader range of wavelengths over which optical signals can be transmitted, but also to increased transmission capacity. One might therefore say that devices that reduce wavelength imbalance are indispensable in optical communications.
Hitherto, the wavelength imbalance has been reduced by inserting an optical equalization filter into the optical fiber transmission path. However, in cases where the wavelength dependence of the optical amplifier changes due to changes in the optical signal strength or optical amplifier gain, an optical equalization filter is unable to track these changes dynamically.
To deal with this problem, the use of variable-gain optical equalizers has been investigated. FIG. 11 shows a conventional example of a variable-gain optical equalizer. In this variable-gain optical equalizer, the incident light is split by a demultiplexer 51 into light at individual wavelengths λ1 through λn, and the light at each wavelength is individually attenuated by a variable optical attenuator 50 and then recombined by a multiplexer 51′. In this figure, 15 is an optical coupler that is wavelength-independent at least over the wavelength region of the optical signal, 16 and 16′ are optical amplifiers, 14 is a spectrum monitor, and 52 is a circuit that drives variable optical attenuator 50.
Japanese Patent Application Kokai Publication No. 2000-199880 discloses a technique whereby a plurality of filter modules are connected in series and the wavelength characteristics of each filter module are controlled in order to maintain constantly flat wavelength characteristics by tracking changes in the wavelength characteristics. (This publication is referred to as “Reference 1” below.)
A general description of the abovementioned variable-gain optical equalizers is presented below, along with a description of their problems.
Variable-gain optical equalizers can be broadly classified into the following types:                (1) variable-gain optical equalizers in which the optical signal is split into different wavelengths by a demultiplexer, each wavelength is processed in parallel by an optical attenuator element whose light intensity attenuation factor can be varied for this wavelength, and then the wavelengths are recombined by a multiplexer, and        (2) variable-gain optical equalizers that do not use a demultiplexer but comprise a plurality of cascade-connected attenuation elements whose attenuation factors and the wavelength characteristics thereof can both be varied.        
The configuration illustrated in FIG. 11 is a typical example of a variable-gain optical equalizer of type (1). This type can also be implemented with a single demultiplexer/multiplexer by using an optical circulator and reflective optical attenuator elements. Either way, since this type requires variable optical attenuators equal in number to the number of wavelength channels, it is not possible to avoid the increased costs associated with the elevated number of components. Another problem is that large insertion losses are incurred because the optical signals are subjected to a multiplexing process after they have been split apart. Furthermore, to ensure that the wavelength gain equalization is controlled stably, each optical attenuator element must be controlled by monitoring the light intensity in each wavelength channel, resulting in problems due to the increased complexity of the control system as the degree of multiplexing increases.
A fundamental technique for a variable-gain optical equalizer of type (2) is a technique for flattening the wavelength characteristics of the optical gain by employing a plurality of cascade-connected optical attenuator elements. However, these optical attenuator elements have consisted of elements such as transmissive Fabry-Perot resonators in which the attenuation factor and wavelength characteristics thereof are fixed. Reference 1 proposes a technique that makes it possible to adapt these to fluctuations in the optical amplifier gain characteristics and the spectrum of the input light.
And methods that might be used to modify the attenuation factor wavelength characteristics with a plurality of optical attenuator elements include using optical attenuator elements corresponding to the Fourier series components of the attenuation factor wavelengths, or a non-linear fitting application technique. Also, instead of Fabry-Perot resonators, the optical attenuator elements might be Mach-Zehnder filters or gratings or the like.
But regardless of whether Fourier expansion or non-linear fitting is used, in order to construct a gain equalizer as a variable device that can adapt to fluctuations of the input light, the two factors of attenuation factor and the wavelength characteristics at which the cascade connected optical attenuator element operates must be subjected to variable control, and specific devices that have hitherto been proposed to achieve this have therefore employed an active configuration where a Mach-Zehnder filter, Fabry-Perot resonator or grating is combined with an optical amplification medium, and all of these examples are configured with a variable amplification element according to Reference 1. However, no specific mention can be found relating to the method for controlling the wavelength characteristics of the attenuation factor of the optical attenuator elements, or the method for controlling the gain characteristics and phase characteristics independently. Also, the elements in these configurations are all transmissive.